Sam Taylor-Johnson’s forthcoming big screen adaptation of literary sensation Fifty Shades of Grey will be the raunchiest mainstream movie for more than a decade, with more than 20 minutes of sex scenes across its 100-minute running time, according to the Sunday Times.

Earlier reports from the annual CinemaCon event in Las Vegas last March suggested Taylor-Johnson’s film wasn’t particularly hardcore, with the tone deemed “more romantic than sexual”. Last month the movie escaped the most restrictive NC-17 rating from US censors in a move which appeared to confirm the film was set to water down the BDSM tilt of EL James’s infamous knee-trembler.

But the Times reports that James herself has stepped in to persuade Taylor-Johnson to “spice up” the £27m film’s sex scenes and cites data from film-nudity website Mr Skin as proof. According to the site, the 100 mainstream films rated 18 in the UK and R in America in 2014 had less sex-scene time put together than Fifty Shades .

By comparison famously salacious movies such as Bernardo Bertolucci’s 1972 drama Last Tango in Paris and Stanley Kubrick’s 1999 thriller Eyes Wide Shut featured just nine minutes and two minutes of sex respectively. Fifty Shades of Grey comes out behind 2004 Michael Winterbottom drama 9 Songs, however, which featured sex for a whopping 41 out of 75 minutes.

Taylor-Johnson’s film, which stars Jamie Dornan and Dakota Johnson as kinky billionaire Christian Grey and willing submissive Anastasia Steele, is being released in time for Valentine’s Day on both sides of the Atlantic. Dornan has spoken of visiting real sex dungeons as research for the role, but he also suggested in an interview with the Guardian in November that the film version would “appeal to as wide an audience as possible without grossing them out” and confirmed full-frontal male nudity would not be on the table. “There were contracts in place that said that viewers wouldn’t be seeing my ... todger,” said Dornan.

The three books in James’ Fifty Shades of Grey trilogy have sold more than 100m copies worldwide and been translated into more than 50 languages. Studio Universal is expected to make at least three films based on the novels.